1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method and apparatus for selection and assignment of a traffic channel during call set-up and hand-off by evaluating uplink signal strength measurements combined with frequency packing analysis.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is contiguous with multiple adjacent cells to provide continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links (and voice trunks) with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to and from the mobile stations through the base stations and, in general, control operation of the system.
Each cell is allocated the use of a predetermined set of frequencies, wherein each frequency comprises a physical channel supporting a plurality of logical channels (i.e., time slots) therein. The availability of multiple frequencies per cell, with multiple logical channels per frequency, permits base stations to simultaneously handle many telephone conversations with many mobile stations. The frequencies allocated to a cell are preferably spaced apart across the frequency spectrum of the cellular band. This serves to minimize the instances of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular band, allocation of the same frequencies is repeated (i.e., reused) in other cells in a distant part of large service areas with many cells. No adjacent cells, however, are assigned the same frequency. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength. The foregoing precautions serve to minimize the likelihood of co-channel interference caused by reuse of that same frequency in a distant cell.
In spite of the precautions taken by service providers, it is known that instances of adjacent channel and co-channel interference do occur. This interference often adversely affects system operation by, for example, degrading voice or data quality on the traffic channels or interfering with the transmission and reception of control signals on the control channels.
The mobile switching center functions to dynamically assign the traffic channels available in any one cell among the plurality of mobile stations located within the cell area that desire communications. In this regard, the traffic channels comprise time slots in digital time division multiple access (TDMA) formatted frequencies which are allocated to the cell. Commands that assign for mobile station use a certain digital traffic channel assigned to a given cell are transmitted from the mobile switching center to the base station for that cell. The commands are then relayed by the base station to the certain mobile station over one of the channels to direct mobile station selection of the assigned traffic channel for handling the call.
Assignment by the mobile switching center of a particular digital traffic channel in a cell to a particular mobile station for carrying a cellular communication (i.e., a call) primarily occurs in two instances. The first instance is at call set-up when the subscriber activates the mobile station to initiate a call and the system selects for assignment the traffic channel to carry that new call. The second instance is at call hand-off when the subscriber, while engaged in a call, moves from one cell in the service area to another cell, and the system selects for assignment the traffic channel in the new cell that will continue the handling of the on-going call. In either case, it is important that the traffic channel selected for assignment at call set-up or hand-off be of the highest quality possible.
Conventionally, the selection and assignment by the system of a traffic channel to carry a call is performed by randomly choosing a logical channel from the available (idle) digital traffic channels allocated to the cell currently serving the mobile station. Alternatively, the system maintains a record of historic traffic channel use for each cell (normally in the form of a FIFO queue), and the traffic channel selected to carry the call is the channel allocated to that cell which has not been assigned for mobile station calling use in the longest period of time. While each of these selection methods is successful in assigning a traffic channel in a cell to a mobile station for carrying the call, the selection fails to take into consideration whether the traffic channel is the best quality (i.e., least interfered) channel available to carry the call. The prior art selection methods further fail to take into account concerns over minimizing the interference that may subsequently arise following the assignment of the traffic channel, and minimizing the number of frequencies simultaneously in use. In many instances there may be significant interference (often comprising adjacent channel or perhaps more frequently co-channel interference) on the randomly or queue selected traffic channel, or unacceptable interference may result following assignment due to inefficient simultaneous use of frequencies, and another one of the available idle traffic channels in the cell would have been a more appropriate assignment selection for use in carrying the call.
A need then exists for an improved method for assigning traffic channels to carry a call in response to either a call set-up or hand-off occurrence. This method should give consideration not only to whether the traffic channel is the best quality (i.e., least interfered) channel available to carry the call, but also whether a minimum required number of frequencies are being used as is possible to handle traffic needs.